Sickle cell disease (SCD) is an inherited blood disorder associated with severe anemia, vessel occlusion, poor oxygen transport and organ failure. The presence of stiff and often sickle-shaped red blood cells is the hallmark of SCD and is believed to contribute to impaired blood rheology and organ damage. Most existing measurement techniques of blood and red blood cell physical properties require sample contact and/or large sample volume, which is problematic for pediatric patients. Acoustic levitation allows rheological measurements in a single drop of blood, simultaneously eliminating the need for both contact containment and manipulation of samples. The technique shows that the shape oscillation of blood drops is able to assess blood viscosity in normal and SCD blood and demonstrates an abnormally increased viscosity in SCD when compared with normal controls. Furthermore, the technique is sensitive enough to detect viscosity changes induced by hydroxyurea treatment, and their dependence on the total fetal hemoglobin content of the sample. Thus this technique may hold promise as a monitoring tool for assessing changes in blood rheology in sickle cell and other hematological diseases.

The method of exciting shape oscillation of drops to extract material properties has a long history, which is most often coupled with the technique of acoustic levitation to achieve non-contact manipulation of the drop sample. We revisit this method with application to the inference of bulk shear viscosity and surface tension. The literature is replete with references to a “10% oscillation amplitude” as a sufficient condition for the application of Lamb's analytical expressions for the shape oscillations of viscous liquids. Our results show that even a 10% oscillation amplitude leads to dynamic effects which render Lamb's results inapplicable. By comparison with samples of known viscosity and surface tension, we illustrate the complicating finite-amplitude effects (mode-splitting and excess dissipation associated with vorticity) that can occur and then show that sufficiently small oscillations allow us to recover the correct material properties using Lamb's formula.

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Acoustic tweezing rheometry is an innovative technology for low-volume non-contact rheological analysis of complex fluids characterized by increased sensitivity and accuracy as compared to traditional contact techniques. In this method, a small drop of a fluid sample is levitated in air by acoustic radiation forces and its viscoelasticity at different time instants is measured from drop shape changes. The acoustic tweezing rheometer operates in two different modes: quasi-static and oscillatory. This presentation focuses on the oscillatory technique in which the sample drop is forced into freely decaying shape oscillation by transient modulation of the standing acoustic field. Images of oscillating drops acquired by a high-speed camera are analyzed by a custom MATLAB code to obtain the shape amplitude vs. time curves and then the decay factor and resonance frequency of drop shape oscillation. Dynamic viscoelasticity of the sample (viscosity, relaxation time, and elastic modulus) was measured by applying the experimental data to the analytical formulae, derived from normal mode analysis of drop oscillation for viscoelastic fluid (Maxwell model) and viscoelastic solid (Kelvin-Voigt) materials. Using the oscillatory technique, we measured changes in blood viscoelasticity during coagulation and showed that the Kelvin-Voigt model leads to physically consistent results on rheology of coagulating blood.